When bits of data are transmitted over a system, the bits are sometimes corrupted. Consequently, a receiving unit that receives the data is unable to decode the corrupted data bits, or improperly decodes one or more of the corrupted data bits. Various error detection techniques are employed to detect bit errors. One such technique is known as the cyclic redundancy check (CRC) technique. The CRC technique produces a binary checksum using bits of data to be transmitted and appends the checksum to the data bits and transmits the bits of data and the bits making up the checksum together over the system. The receiving unit receives the checksum and the data and uses the checksum to determine whether the data bits contain any bit errors.
In systems that use what is known as the automatic repeat request (ARQ) protocol to correct bit errors, blocks of data are discarded if the results of performing the CRC technique indicate that one or more bits of a block are incorrect. Block failures are signaled to the transmitting unit, which, in turn, retransmits the failed data block.
In systems that use what is known as a HARQ protocol, blocks that fail CRC are not discarded. Rather, information (e.g., hard bits, soft information (e.g., log likelihood ratios), baseband samples, or the like) from the failed block transmissions are stored and subsequently combined with information from previous transmissions of the same block in order to improve error performance. However, this requires that the receiving unit have the knowledge to determine which information from which data block retransmissions can be combined. Normally, this knowledge is signaled at the logical link control (LLC) layer, which is coded along with the payload information. This means that the receiving unit cannot determine which data blocks to combine unless it can decode the data blocks.
An alternative to signaling this knowledge at the LLC layer is to signal block ordering information at the media access control (MAC) layer. To signal this information directly at the MAC layer requires a significant amount of signaling overhead. For example, in the existing Telecommunications Industry Association (TIA)-902 standard, a 12-bit segment/sequence number is used to uniquely identify each data block at the LLC layer, and a plurality of data blocks are transmitted in each slot. Each slot can have up to six blocks. Therefore, to signal this block ordering information at the MAC layer to support HARQ requires that seventy-two additional bits are added to a MAC header block (MHBK) which equates to one twelve-bit sequence number for each of the possible six blocks in the slot. This translates into a loss in raw throughput of 7.2 kilobits per second (kb/s), which does not even take into account that duplicate segment/sequence numbers can exist on the outbound link differentiated by the receiving unit address, which would require an even larger numbering field for the MHBK, thereby further limiting throughput.
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